The present invention relates to the field of integrated circuits, and, more particularly, to an integrated voltage generator for generating a voltage ramp with a relatively small slope.
The increasing number of functions implemented in an integrated circuit, with the consequent reduction of the number of available pins, leads to the development of circuits requiring fewer and fewer external components. A function difficult to implement in a fully integrated form is the generation of voltage ramps with relatively small slopes, for example, on the order of several hundreds of milliseconds. Such voltage ramps are used as start-up ramps in various devices, e.g. in devices for audio applications, to slow down the transitions from the MUTE condition to the PLAY condition and vice versa.
For audio applications, at the assertion of a MUTE/PLAY transition, it is necessary to generate a voltage ramp with a positive slope, as indicated in FIG. 1, while in the case of switching from PLAY to MUTE it is necessary to generate a voltage ramp with a negative slope. Such a slow voltage ramp allows gradual transitions between the two operating conditions.
Conceptually, voltage ramps can be generated by forcing a charge current on a capacitor and outputting the voltage produced on it, as illustrated in FIG. 2. In integrated circuits, the generation of voltage ramps lasting one hundred milliseconds and over poses problems, because large capacitances, which are difficult to integrate, are needed.
For example, if a voltage ramp switching from a 0V to a voltage of 5V in a time of 10 milliseconds or even in a longer time is required, as frequently required in audio applications, the current generator I of FIG. 2 should force a current lower than 50 nA in the capacitor C. But, due to silicon area requirements and the corresponding costs, the integration of capacitances greater than 100 pF is too expensive.
Generation of such small currents which are compensated for the process spread and for temperature variations is very difficult, making devices of the desired precision and reliability hardly feasible. A way to avoid such a problem is to use external capacitors, but this gives up the advantages of a fully integrated approach. Drawbacks of the known approaches are either relatively complex hardware and software, a greater number of external components, an increment of cost and an intrinsic reduction of reliability of the system.
It is an object of the present invention to provide a fully integrated circuit for producing a slow voltage ramp with a precisely controllable relatively small positive or negative slope. The circuit of the invention produces a polylinear output voltage that can be approximated to a linear voltage ramp, using a generator of a voltage ramp with a relatively steeper slope.
More precisely, the invention includes an integrated circuit for producing a voltage ramp with a small slope. The integrated circuit includes a circuit for generating a periodic triangular current signal, and a circuit for generating, at the beginning of each period of the triangular signal, a pulse of a certain duration much smaller than the period of the triangular signal. Also, the integrated circuit includes a feedback loop input with the triangular signal and producing the slow voltage ramp on the output node while being controlled by the pulse.
The feedback loop comprises a first hold circuit coupled to the input node through a first switch controlled by the pulse, for producing the voltage ramp on an output node. Also included are a transconductance operational amplifier, whose inputs are respectively coupled to the input terminal and to the output node, a second hold circuit coupled to the output of the transconductance amplifier through a second switch controlled in phase opposition of the first switch, and a resistor of a relatively small value compared to the ratio between the period of the first triangular signal and the capacitance of the storage capacitor of the first hold circuit, which is connected between the output of the second hold circuit and the input node.